The MM_WOM_DONE message is sent to a window when the given output buffer is being returned to the application. Buffers are returned to the application when they have been played, or as the result of a call to the waveOutReset function.
MM_WOM_DONE 
wParam = (WPARAM) hOutputDev 
lParam = (LONG) lpwvhdr 
 This message does not return a value.
  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in mmsystem.h.
Waveform Audio Overview, Waveform Messages, waveOutReset, WAVEHDR